Recently, development of FDD (Frequency Division Duplex) cellular radio communication systems adopting an OFDMA scheme is carried out actively. As a cellular radio communication systems adopting the OFDMA scheme, LTE (Long Term Evolution) is adopted as a worldwide standard.
In a cellular radio communication system, terminals become able to perform communication by accessing a radio network comprised of base stations and a core network. As for a basic function such as phone call, when a terminal sends a service request to a base station, the service is applied by establishing a connection with a terminal attached to the same base station or another base station via the core network. Also in the case of data communication, similarly, when a terminal sends a request for data communication to a base station, exchange of necessary information is performed via the core network. That is, in a cellular radio communication system, a terminal can perform communication only in an area covered by a base station.
A service area covered by a base station is called a cell. Cells are classified into so-called macro cells, pico cells, femto cells, etc. according to cell size. Or, the cells may be called differently according to different support functions. Near the edge of a cell, where communication quality deteriorates, it becomes difficult to keep good communication between a terminal and a base station. A region called a cell edge is known to be produced by two causes: one is that power from the base station attenuates over a long distance and the other is that interference radio waves arrive from a neighboring cell. In an OFDMA cellular radio communication system, it is thought that interference from a neighboring cell becomes dominant. When an interference power level from a neighboring cell becomes comparable to a desired signal level from the home cell, deterioration of SINR (Signal to Interference and Noise power Ratio), which is one of indicators representing communication quality, occurs. A terminal located in the cell edge has a deteriorated SINR and cannot perform communication unless it uses more radio resources than normally required. That is, a cell edge terminal is not only unable to perform high-speed communication and it can also be said that such a terminal occupies radio resources which might otherwise be assigned to another terminal having a good SINR.
Typically, in designing areas by a network operation, cell edges are allocated to land forms where terminals are unlikely to be located. However, in the center of a city among others, in a case where a base station's capacity accommodating terminals is exceeded because of dense population, another base station may be deployed nearby for the purpose of load distribution. In such a case, it is hard to design cells to allocate cell edges to places where fewer terminals may be located. Therefore, it becomes important to improve communication quality in cell edges by optimizing a communication method.
For instance, it is known that adoption of FFR (Fractional Frequency Reuse) is effective. FFR is a technique that splits a frequency band in which radio signals can be transmitted with high power into parts assigned to adjacent cells and covers cell-edge users by using the frequency band parts. If the FFR is applied, assignments of radio resources and power are coordinated according to classification into cell-center terminals and cell-edge terminals. Unless the FFR is used, transmission with equal power takes place in a whole band (f0). Scheduling can be performed without discrimination between cell center (near the base station of the cell) and cell edge, but in the cell edge, large interference occurs between adjacent cells. Thus, in the FFR, by splitting the frequency band (f0) split into bands (f1, f2, and f3), coordinating power that can be output in each frequency band, and appropriately selecting one of combinations (f1, f2, and f3) for each cell, suppression of interference between adjacent cells is carried out.